The field of the invention is nuclear magnetic resonance imaging systems; and more particularly, to coil assemblies employed in such systems to excite the nuclei and receive electrical signals produced by the nuclei.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant g of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but randomly oriented magnetic components in the perpendicular, or transverse, plane (X-Y plane) cancel one another. If the substance or tissue is subjected to a magnetic field (excitation field B.sub.1) which is in the X-Y plane and which is near the Larmor frequency, the net aligned moment (M.sub.z) may be rotated, or "tipped", into the X-Y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the X-Y plane at the Larmor frequency. The practical value of this phenomenon resides in the electrical signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective X, Y and Z axes. The field gradients are produced by a trio of coils placed around the object being imaged. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
The excitation magnetic field, near the Larmor frequency in the radio frequency spectrum, is produced by a separate coil placed adjacent the area of interest in the object being imaged. If an image of substantially the entire object is desired a large radio frequency (RF) coil, often referred to as a "body coil" and extending around the entire object, is used to excite the spinning nuclei and receive the resultant electrical signals. In other situations when only a small portion of the object (such as the head of a medical patient) is to be imaged, a smaller radio frequency coil is placed about that portion of the object. The use of localized coils produce a more homogeneous excitation field within the desired portion.
One common type of radio frequency coil is cylindrical with a conductive loop at each end and axial conductive elements connecting the loops at periodic intervals around their circumference, as described in U.S. Pat. Nos. 4,680,548 and 4,692,705. The coil formed an endless loop transmission matrix that is excited by two signals in quadrature to produce a rotating electromagnetic excitation field within the coil. Because of its shape and appearance, this coil structure is sometimes referred as a "birdcage".
In a typical coil assembly, the radio frequency coil is located within the gradient coils with a relatively small spacing therebetween. The close physical proximity of these different coils results in a significant amount of the energy from the RF excitation field being lost due to impingement upon the gradient field coil structure. This loss shows up as a damping of the quality factor Q of the contained radio frequency coil which, in turn, degrades the normally attainable signal-to-noise ratio of the imaging device. Accordingly some type of RF shielding is usually placed between the radio frequency and gradient coils to preserve the Q of the former coil and consequently the signal-to-noise ratio of the system.
A solid copper shield has been proposed, however, gradient field induced eddy currents would be supported anywhere on such a structure. In addition, it is often desirable to be able to insert probes and other medical instruments through the coil assembly and into the patient being imaged which could not be done with a solid shield. Further in the case of localized RF coils for the patient's head, it is desirable to provide windows through which the patient can see to prevent a claustrophobic feeling during prolonged MRI scanning.